well today i looked in my hanes book to see about my check engine light, i have a 95 nissan altima. anyways i went into the emc, and found out that the knock sensor and the rear o2 sensor is what is causing the chekc engine light to come on, now i was wondering i know the knock sensor is about $170, but how hard is it to install, and and rear o2 sensor how hard is it to install.???????????? or do i even need to fix them, is it worth it? its got 160000 miles on it. do i really need them?